Light emitting diodes (LEDs) are widely used in consumer and commercial applications. Continued developments in LED technology have resulted in highly efficient and mechanically robust light sources arranged to output emissions in the visible spectrum and beyond. These attributes, coupled with the long service life of solid state devices, have enabled a variety of new display applications, and have resulted in use of LEDs in general illumination applications with the potential to replace incandescent and fluorescent lamps.
As is well known to those skilled in the art, a light emitting diode generally includes an active region fabricated from a material having a suitable bandgap such that electron-hole recombination results in the generation of light when current is passed through the device. In particular, materials in the Group III nitride material system, such as GaN, InGaN, AlGaN, InAlGaN, etc., have been proven useful for generating blue, green, and ultraviolet light with relatively high efficiency.
Group III nitride based LEDs may be fabricated on growth substrates (e.g., silicon carbide substrates) to provide horizontal devices (with both electrical contacts on a same side of the LED) or vertical devices (with electrical contacts on opposite sides of the LED). The growth substrate may be maintained on the LED after fabrication, or may be removed such as by chemical etching, grinding, polishing, laser lift-off, or other suitable processes. Removal of a growth substrate may beneficially reduce a thickness of the resulting LED and/or reduce a forward voltage through a vertical LED. A horizontal device (with or without the growth substrate), for example, may be flip chip bonded (e.g., using solder) to a carrier substrate or printed circuit board, or wire bonded. A vertical device (with or without the growth substrate) may include first and second terminals bonded to a carrier substrate or printed circuit board.
Attempts to improve the light output of Group III nitride based devices have included providing differing configurations of the active regions of the devices. Such attempts have, for example, included the use of single and/or double heterostructure active regions. Similarly, quantum well devices with one or more Group III nitride quantum wells have also been fabricated. While such attempts have improved the efficiency of Group III nitride based devices, further improvements may still be achieved.
One problem that has been experienced with Group III nitride devices is “current droop,” a phenomenon in which light output increases with current density up to a point, and then begins to level off. Thus, device efficiency may drop off at higher currents. Without being bound by any particular theory, it is presently believed that current droop may be the result of one or more factors, including saturation of hole injection and/or inefficient (i.e., non-light generating) electron-hole recombination at higher device currents. A similar or related problem that has been experienced with Group III nitride based devices is “thermal droop,” a phenomenon in which light output decreases with elevated operating temperature. Such phenomenon may be attributable at least in part to the fact that the probability of non-radiative (i.e., non-light-emitting) recombination of electrons and holes increases with temperature. Reductions in luminous flux may cause undesirable and perceptible color shifts at elevated temperatures, particularly in lighting devices with multiple emitters.
Additional considerations that impact Group III nitride device design and/or operation are efficiency and forward voltage. Adjustment of certain parameters that may beneficially enhance charge confinement in quantum wells may also result in detrimental increases in forward voltage (i.e., the minimum voltage difference between the anode and cathode required to conduct electricity and activate a LED). Balancing such considerations can complicate Group III nitride based device design.
A need exists for Group III nitride based devices with improved performance.